Ultrasound imaging is widely used in medical applications to non-invasively observe structures within the human body, such as cardiac structures, the vascular system, the fetus, the uterus, the abdominal organs and the eye. In a typical imaging system, short bursts of ultrasound energy are directed into a patient's body with a handheld transducer. The returning reflected energy is received by the same transducer. The signals representing the reflected energy are processed and formatted into a video image of the target region. Phased array scanning techniques are commonly used.
In real-time diagnostic medical ultrasound systems, two-dimensional image quality plays a key role in providing diagnostically significant information for accurate patient diagnosis. However, due to the physical properties governing the generation of an image based on acoustic propagation and reflection of ultrasound energy, state of the art images are affected by various artifacts and degradations. Specifically, clutter and speckle artifacts are observed in the images. Clutter appears as a more or less stationary area of cloudiness formed by small areas of excess, undesired ultrasound energy. Speckle is characterized by small grainy areas where, due to the coherent nature of acoustic image formation, no return signal energy is detected, despite the fact that there is a reflecting medium in the field of view.
Various techniques have been proposed in the prior art for reducing clutter and speckle artifacts in ultrasound images. These approaches generally involve combining images obtained at different times and/or in different frequency bands. See, for example, U.S. Pat. No. Re. 35,148 issued January 23, 1996 to Lizzi et al., which describes a frequency compounding technique. A broadband received signal is applied to separate bandpass filters. The outputs of the filters are detected and summed.
A two-dimensional median filter to reduce speckle artifact in ultrasound imaging is disclosed in U.S. Pat. No. 5,409,007 issued Apr. 25, 1995 to Saunders et al.
A technique for speckle reduction in ultrasound imaging using a two-dimensional array of transducer elements is disclosed in U.S. Pat. No. 5,653,235 issued Aug. 5, 1997 to Teo.
Spatial compounding is another prior art technique for reducing speckle in ultrasound images. In spatial compounding, an object is imaged from two or more points in space, and the resulting images are combined to form a single image. If the points in space are sufficiently distant from one another, the speckle patterns produced are not correlated with each other.
Prior art techniques for reducing clutter and speckle artifacts have exhibited limited improvement in image quality. Accordingly, there is a need for additional techniques for enhancing image quality, which may be used separately or in combination with known techniques.